This will also free up a lot of leg/book room. Not that it maters to me but I've had some passengers who could have used a few extra inches.
Here is a sketch up.

5.5cuft gross, roughly 4.8 after amp, driver, bracing displacement. It will be stuffed with 4lbs of poly fill. Basically doubleing everything from my last mag earsex enclosure (one mag sealed). I'm thinking about making it a tad bit longer, moving the subs out a bit more, and flushmounting the sundown horazontally in between them. I'll have to think about that look. It is either there or on top.

I'll keep you updated on progress once I actually start it.